1. Field of the Invention
This invention relates to a resin composition for the manufacture of mulching films for agricultural use. The resin composition comprises polyisobutylene oxide resin, and either (1) polyolefin resin, or (2) a mixture of polyolefin resin and polydiene resin, and it is characterized by the property that mulching films made of the composition degrade, under the action of sunlight, into a pulverized condition within a selected period of time. Further, the composition can additionally contain an ultraviolet light sensitizer or ultraviolet light absorber.
2. Description of the Prior Art
In the cultivation of agricultural crops, it is known to use mulching films made of synthetic resins, for example, polyethylene, polyvinyl chloride, and polyethylene-polyvinyl acetate copolymers. These films keep soil from lowering in temperature, protect plants from frost, and inhibit evaporation of moisture in the soil, thus promoting the growth of the plants satisfactorily in a shorter period in comparison with the time when mulching films are not used. Additionally, when such films are pigmented so as to be substantially opaque, they inhibit the growth of weeds.
If these mulching films, however, remain in unitary film form at the time of harvest, they are obstacles to convenient harvesting of the crops, for example, by interferring with the movement of a harvesting machine. Moreover, during the hot summer season, they may adversely affect the growth of plants because they absorb the sun's heat and may cause the soil eventually to dry excessively. It is necessary, therefore, to remove the mulching films by hand carefully in such a way as not to injure the crops or plants. This removal operation is very troublesome and expensive. Further, the discarded films are a nuisance to dispose of, in an incinerator, because they generate high temperatures and/or emit corrosive gases and thereby cause damage to the incinerator.
In order to dispense with the trouble of removing the used films from the field and of disposing of them in an incinerator, there have been proposed methods for the manufacture of polyolefin resin films containing a light-degradation accelerator which speeds up photolytic decomposition of the films by sunlight, so that the films become friable within a reasonable period of time. The methods known in the prior art include Japanese Patent Publication Nos. 13707/1970 and 33020/1973, British Patent Specification No. 1 034 076, all for polyolefin resin, and also U.S. Pat. No. 3,590,528 for polybutene-1 film. These films disclosed in the prior art are not fully satisfactory because, for example, some of them degrade to fast to keep the field warm enough, thus failing to accelerate the growth of crops; others degrade into large fragments which are then scattered by wind and rain. As a result, these large fragments become buried in the soil and remain unchanged in form for many years, thereby making subsequent plowing of the field by machine extremely difficult.
We previously discovered that a composition containing polyisobutylene oxide resin, when contacted by ultraviolet light, decomposes initially into powder and eventually into volatile organic compounds, which then finally evaporate and dissipate. See U.S. Pat. No. 3,850,855.
Such a composition of polyisobutylene oxide resin and polyolefin resin is disclosed in Japanese Patent Publication No. 5216/1965, in which, however, the use of the composition for mulching films is not mentioned.